1. Field of the Invention
The present invention relates to a converging dual-beam optical semiconductor wafer and magnetic disk edge detection system for detecting the presence of small, specular surfaces. More particularly, the present invention relates to a detection system which includes an optical sensor having at least 2 light sources, preferably lasers, and at least 2 light detectors spatially oriented such that the beams from the light sources define a plane and converge at a single point external to the device, and the light detectors are arranged such that the specular or reflective surface to be detected interrupts the beam at or near the focal point causing the light to be reflected backwards towards the sensor for detection by the light detectors.
2. Brief Description of the Prior Art
Since the construction of the first laser in 1960, the use of lasers in science, industry, and commerce has steadily risen. The semiconductor and magnetic disk storage device industry is no exception. Lasers are being used in these industries to perform processes such as intricate etching, and cutting operations on semiconductor wafers, and to measure physical product attributes, such as the film thickness of a lubricant layer on a substrate.
A proposed use for the device of the present invention is for the detection of the perimeter edge of semiconductor wafers or magnetic memory disks particularly where such wafers or disks are spaced in close proximity to one another either for processing or transport. Semiconductor wafers and small magnetic disks are generally racked or mounted vertically on their edges and stacked horizontally in plastic cassette carriers or boats. Each carrier contains many parts next to each other with a small separation between each part. Detecting the edge of a wafer or disk permits accurate positioning information to be obtained allowing automated handling equipment to access and remove individual parts for processing without damaging adjacent parts in the carrier. Further, the device of the present invention can detect improperly aligned parts, missing parts, double-wafers or double-disks (i.e., wafers or disks mounted with no spaces between them) alerting the technician or automated equipment to possible defective parts, or to pass over the defective parts to prevent further processing.
Work-in-progress inventories of unfinished product require that an accurate count be made of the aggregate number of parts in the special cassettes or boats throughout the process line. This count is then used to valuate the unfinished goods in the process line, as well as to suggest improvements to queuing and parts flow to reduce costs associated with excess inventory. Similarly parts flow tracking to identify process bottlenecks and low yield processes is essential in production hardware planning and process improvement efforts. Counting the parts throughput at a particular operation and comparing it to the throughput required to meet market demand assists the capital planner in determining the number of process machines required to meet that demand. Further, by comparing the count from operation to the next, those operations or machines with lower production yields may be identified for engineering attention.
Driven by a nearly exponential increase in demand for certain semiconductor and magnetic disk products, and the stringent clean room requirements necessary to manufacture these products, companies look to increased automation of their processes to overcome the contamination exposure when human operators and technicians are present in the process line. Automation is necessary to handle the sheer number of parts which must be counted, processed, and tested. However, automated parts handling requires that parts be sensed, i.e., that they are, in fact, present. Recognizing that parts sensing is essential for counting, for positioning for automated handling and parts transfer, and for identifying defective parts due to defective positioning in the carrier or boat, a fast, accurate, and reliable sensing system is essential to an automated parts handling system. However, the extremely thin, compoundly curved edges of semiconductor wafers and magnetic disks have, until now, represented a significant challenge in developing edge sensing devices capable of rapidly and accurately sensing these edges.
Sensing devices currently being used to count semiconductor wafers include a "through beam". Through-beams do not detect the edge of the wafer per se, rather, a light beam is passed through the cross-section of the parts carrier and moved longitudinally down the parts carrier. A count is made as the wafers or disks in the carrier interrupt the beam. However, this device is difficult to align and, generally, must be dedicated to a particular parts cassette. Further, the through beam is unable to detect "double-stacking"; i.e., where two wafers or disks have inadvertently been mounted in the parts carrier so that their adjacent faces are in contact. Double stacking invariably causes defects, such as scratches, on the precision surfaces of these products resulting in lower process yields and increased costs due to rejected parts. Ideally, early detection of double stacking is desirable to prevent further costly processing of these defective parts and to help identify which process step is the cause of the double-stacking. Through-beam parts detection will not be able to identify double stacked parts in those carriers where the parts are tilted or slightly askew in their slots. As semiconductor wafers and magnetic disks are very thin and the slot in the carrier is generally of a design that does not support the wafer or disk equally around its circumference the wafer or disk will sit slightly tilted in that slot. The tilted wafer or disk presents a wider profile to the through-beam than that of a perfectly aligned wafer or disk. This wider profile may be erroneously interpreted by the through-beam system as a double stacking occurrence.
Another process related error which may result in product defects is "cross-slotting." Cross-slotting occurs when a semiconductor wafer or magnetic disk is positioned in the parts carrier such that one edge of the wafer or disk in contact with the carrier is in the wrong retaining slot in the carrier. This results in the position of the wafer or disk in the carrier to be skewed or askant. Through beams are unable to detect such a condition. Detection of this process error requires comparing the position of the two side or laterally positioned edges of the wafer or disk to determine if the position of one edge along the longitudinal coordinate is different from the longitudinal coordinate of the other edge. Through-beam systems can detect a profile of the wafer or disk along the top to bottom axes of the parts carrier and for the reasons given above for double stacking, through-beams are unable to accurately detect parts which are in a skewed position. Consequently, through-beams would be unable to sense a side-to-side positional error such as that caused by cross-slotting. On the other hand, the accurate positioning capability of the dual beam sensor system of the present invention would merely require that both lateral edges of the wafers or disks in the carrier be scanned and the position of the edges of any parts be compared. Any relative differences in edge to edge spacing can be determined and the appropriate action taken.
Another parts detection scheme involves the use of a fiber optic light guide brought in close proximity to the position where the perimeter edge of the part is anticipated to be. This scheme is used extensively in the 10 inch and 14 inch magnetic disk manufacturing industry where large motorized disk carriers or "stackers" are used and the disks are mounted horizontally and stacked vertically. These stackers require precise alignment for docking into a receiving bay. The fiber optic sensor directs light towards the anticipated location of the perimeter edge of the top disk in the stacker. The fiber optic sensor detects the presence of the top disk in the stacker by receiving the reflected light back into the fiber optic cable with the reflected light being sensed by an optical sensor. However, this system requires that the terminal end of the fiber optic cable be in extreme close proximity to the edge of the top disk and that the incident light from the optic cable impinge at a 90.degree. angle to the tangential surface of the edge of the disk. In addition, the edge of the disk must be thick enough so as to present as flat a surface as possible to the fiber optic light in order to provide enough surface to reflect back a sufficient amount of light to trigger the sensor. Thinner disks or disks having a compoundly curved edge will not reflect sufficient light directly back to the fiber cable and, therefore, the sensor will not detect the disk. In order to maintain such close proximity, the sensor is rigidly affixed to either the stacker or the receiving bay, thus precluding its use for rapid parts counting. This unreliability could result in process throughput deterioration because the fiber optic sensor erroneously senses there are no more parts to process causing the process to stop. Alternately, the stacker may continue indexing upward despite the top disk not having been sensed and removed causing a "double-disk" to occur as the unsensed disks falls back onto the next disk being indexed. In either case, such unreliability will require that an operator or technician be present to continually monitor production processes, thus negating the reasons for installing automated parts handling. This scheme is further limiting since the close proximity and the 90 degree angle of incidence required by the device precludes rapid scanning across the length of the carrier for a rapid parts count.
On the other hand, the spatial relationship of the sources and the detectors of the present invention permit sensing the presence of a reflective surface even where the angle of incident relative to the reflecting surface deviates greatly from 90 degrees thus enabling detection of compound curved reflective surfaces. Even greater deviations are possible for surfaces which have a diffusively reflective surface. Further, close proximity of the part being detected is not a requirement and there is significant latitude in the distance the part may be from the focal point of the converging beams and still be detectable. The light sources may be modulated to improve rejection of spurious signals caused by ambient light and to reduce the response time of the dual beam sensor.
While dual beam scanners are relatively common in the art, dual beam detection systems are conspicuously missing in the art. The background art reveals that scanner technology is generally directed to those situations where the surface to be scanned is presumed to be present and the purpose of the background art is to merely read information or indicia contained on that scanned surface. The present invention, however, makes no such presumption, its purpose being to detect, or determine, if a reflective surface is, in fact, present.
The Shepard et al series of patents (U.S. Pat. No. 4,409,470 et seq.) are directed to a portable laser scanning device for reading bar codes. The device includes a pulse modulated laser, a scanning means, and a detector. The scanned laser beam is directed to a reference plane outside of the device where the bar code is to be read. If a surface having a bar code is present, the laser light is reflected back towards the device wherein the signal is processed to generate data describing the bar code symbol. Shepard does not teach using dual converging beams. Further, the purpose of the Shepard invention is to read indicia from a target surface whose presence is presumed.
The Buczek reference (U.S. Statutory Invention Registration No. H933) is directed towards a laser range finder. The device is a dual beam, amplitude modulated laser transmitter/receiver and includes dual lasers, each modulated to a different frequency. Upon superimposing the two beams, the intensity of each beam is modulated at the laser difference frequency. When the laser frequency of one of the sources changes, a signal is generated which may be used for absolute range measurements. Buczek does not use converging laser beams. Further, the purpose of amplitude modulation is for generating fine doppler shifts rather than for rejection of spurious ambient signals. Buczek reveals information regarding a target presumptively in the path of the laser beams, i.e., the distance to a target already known to be present. As in Shepard the presence of the target is presumed.
The Kramer et al patents (U.S. Pat. Nos. 4,786,126 and 4,826,268) are principally directed towards a twin laser scanning device suitable for use in high speed laser printers for reprographic image production. The device includes pulse modulated, twin collinear laser beams. The beams are scanned across an image surface using a rotating diffraction grating based deflector element. Here, modulation of the laser beams is related to the formation of high density half-tone images and not for spurious signal rejection. The device does not include optical detectors, nor do the laser beams converge to a single point. Kramer also describes using a light emitting diode and detector as an image surface motion detector to measure indicia located on the specular surface of a rotating drum. The indicia causes interruptions in the reflective surface and are used to create a control signal. As in Shepard and Buczek, Kramer presumes that there is a surface present to be scanned.
Shambaugh (U.S. Pat. No. 5,231,463) is directed to using laser Doppler velocimetry to measure the filament velocity in the fiber stream of a melt blowing apparatus. A single laser beam is split into two separate beams, with one beam passing through a Bragg cell causing a frequency shift in that beam. Both beams are optically transferred to a backscatter probe from which they exit. The beams converge at a single point within the fiber stream. The convergence of the two beams form a measuring volume such that when a fiber crosses the measuring volume, optical impulses are created and subsequently detected by a detector in the backscatter probe. The impulses are further processed to provide information as to fiber velocity and other data. Although Shambaugh uses a single laser to generate dual converging laser beams, these beams are frequency shifted using a Bragg cell and narrowly separated. Shambaugh's dual beams are used for laser doppler velocimetry which, again, presumes the presence of a surface (here the surface of a falling fiber), and the attribute being measured here is mass flow.